1. Field of the Invention
The present invention relates to a method for retaining the activity of molybdenum-containing metallic oxide catalysts which are used, for example, for the oxidation (including ammoxidation and oxidative dehydrogenation) of organic compounds that is conducted by means of gas phase fluidized-bed reaction at high temperatures.
2. Related Art
It has been known that molybdenum-containing metallic oxide catalysts are useful for the oxidation (including ammoxidation and oxidative dehydrogenation) of organic compounds. These catalysts are used, for instance, for producing acrolein or acrylic acid by means of the oxidation of propylene; for producing acrylonitrile by means of the ammoxidation of propylene; for producing methacrolein or methacrylic acid by means of the oxidation of isobutene or tert-butanol; for producing methacrylonitrile by means of the ammoxidation of isobutene or tert-butanol; for producing acrylonitrile by means of the ammoxidation of propane; for producing methacrylonitrile by means of the ammoxidation of isobutane; for producing formaldehyde by means of the oxidation of methanol, methylal or the like; for producing hydrogen cyanide by means of the ammoxidation of methanol; and for producing butadiene or the like by means of the oxidative dehydrogenation of n-butene. In addition, the catalysts are also used for the oxidation or ammoxidation of alkyl aromatic hydrocarbons or alkyl heteroaromatic compounds.
As the molybdenum-containing metallic oxide catalysts for use in the above-described reactions, there can be mentioned those which are described in Japanese Patent Publication No. 3563/1961 (U.S. Pat. No. 2,941,007), Japanese Patent Publication No. 27490/1972 (U.S. Pat. No. 3,959,384), Japanese Patent Publication No. 5870/1971 (U.S. Pat. No. 2,904,580), Japanese Patent Publication No. 33888/1976 (U.S. Pat. No. 4,503,001), Japanese Patent Laid-Open Publication No. 47319/1974 (U.S. Pat. No. 3,898,267), Japanese Patent Laid-Open Publication No. 16887/1973 (U.S. Pat. No. 4,036,870), Japanese Patent Laid-Open Publication No. 257/1990 (U.S. Pat. No. 5,049,692), Japanese Patent Publication No. 35400/1976 (U.S. Pat. No. 3,911,089), Japanese Patent Publication No. 39005/1977 (U.S. Pat. No. 3,801,670), Japanese Patent Publication No. 44249/1985, Japanese Patent Laid-Open Publication No. 223812/1995, Japanese Patent Publication No. 14659/1981 (U.S. Pat. No. 3,803,204), Japanese Patent Laid-Open Publication No. 111565/1977 (U.S. Pat. No. 4,055,511), and the like. Catalysts containing both molybdenum and tellurium have also been proposed, and are described in Japanese Patent Publication No. 41583/1987 (U.S. Pat. No. 4,446,328), Japanese Patent Publication No. 41584/1987 (U.S. Pat. No. 4,446,328), Japanese Patent Publication No. 41585/1987 (U.S. Pat. No. 4,446,328), Japanese Patent Laid-Open Publication No. 70922/1974 (GB Patent No. 1,415,766), Japanese Patent Laid-Open Publication No. 38330/1980 (U.S. Pat. No. 4,278,614), Japanese Patent Publication No. 38424/1983 (U.S. Pat. No. 3,969,390), Japanese Patent Publication No. 38425/1983 (U.S. Pat. No. 3,928,409), Japanese Patent Laid-Open Publication No. 114560/1982, Japanese Patent Laid-Open Publication No. 130549/1982 (GB Patent No. 2,090,156), Japanese Patent Laid-Open Publication No. 228950/1989, Japanese Patent Laid-Open Publication No. 118051/1992 (U.S. Pat. No. 5,132,269), Japanese Patent Publication No. 16971/1980, and the like.
Although these molybdenum-containing metallic oxide catalysts have excellent properties, it is known that the molybdenum component is lost from many of the catalysts especially when the reaction temperature is high, thereby causing various troubles such as deterioration in the properties of the catalysts, and the deposition of molybdenum on the inside of the reaction system (J. Bulten, J. Catal., 10, 188-199 (1968); G. P. Wing, L. B. Sis, J. S. Wheeler, J. Catal., 38, 196-205 (1975); I. Nicolau, A. Aguilo, P. B. DeGroot, 4th Inter. Conf. Chem., Uses of Molybdenum, 1982, 234-240; etc.).
Molybdenum component is considered to be lost in the following way: molybdenum trioxide in the catalyst reacts with water in the reaction gas to yield volatile MoO.sub.2 (OH).sub.2, and thus is lost from the catalyst.
To solve this problem, studies were made with the aim of decreasing the amount of free molybdenum trioxide by modifying the composition of the catalyst, thereby converting the free molybdenum trioxide into molybdates. However, this is not necessarily satisfactory because it is difficult to make the results of reaction and the life or properties of the catalyst compatible with each other.
Further, studies were also made to lower the reaction temperature. Regarding fixed-bed reaction, those catalysts which are active even at low temperatures were searched, and prevention of the occurrence of hot spots was also tried. The problem was thus solved to some extent in the oxidation of olefins. However, in those reactions which are carried out at higher temperatures, for example, in ammoxidation reaction of an olefin, the problem of the loss of molybdenum component tends to become an issue more often.
The loss of molybdenum component is thus considered to be unavoidable to some extent, so that a method is usually adopted such that the activity of a molybdenum-containing catalyst is retained by replenishing the molybdenum component during reaction. When the reaction is fixed-bed reaction, there have been used, as the method of this type, those methods which are described in Japanese Patent Publication No. 1848/1966, British Patent Publication No. 814,075, Japanese Patent Laid-Open Publications No. 193136/1984 and No. 10799/1995 (U.S. Pat. No. 5,602,280), etc.; and when fluidized-bed reaction, those methods which are described in Japanese Patent Publication No. 57422/1983, German Patent Publication No. 3,311,521, Japanese Patent Laid-Open Publication No. 301051/1993, etc. All of these methods are such that molybdenum oxide or a molybdenum compound, or molybdenum oxide or a molybdenum compound supported on an inert carrier, or a molybdenum-component-enriched catalyst is added to the reaction system in order to make up the molybdenum component which has been depleted from the catalyst used. Although the catalytic activity can be retained to some extent by these methods, they are troublesome because it is necessary to continuously and repeatedly replenish molybdenum component in a short period of time. In addition, it is difficult, especially in the case of fix-bed reaction, to replenish molybdenum component so that the distribution of the molybdenum component deposited on a catalyst bed will be a desired one. Moreover, there is such a possibility that sublimed molybdenum component is deposited and accumulated in a low-temperature area in the reaction system to cause various troubles, and the replenishment of molybdenum component promotes this tendency.
Thus, in this technical field, there has been a serious demand for developing a molybdenum-containing metallic oxide catalyst from which molybdenum component is not readily lost during reaction and whose excellent activity can be retained for a long period of time without replenishing the molybdenum component, or by replenishing an extremely small amount of the molybdenum component even if the replenishment of the same is needed.